Typically, there are two types of keypads used in radiotelephones. In the first type of keypad, the keys are directly attached to the circuit board internal to the radiotelephone. In the second type of keypad, the keypad is a separate entity from the circuit board contained within the radiotelephone. The keypad and the circuit board are coupled together via wires.
The keypad circuit is used in a keypad to detect activation of one or more keys. The typical circuit contains a multi-layer grid of wires. When a key is activated, the two wires corresponding to the row and column in which the key is located are electrically ground. Each wire is then polled to check for a grounded signal. From the results, the controller can determine which keys are activated.
As a reduction in size and flexibility of design in radiotelephones becomes more prominent, efficient use of space and unique shapes become more important. In the first type of keypad, the keypad utilizes a relatively large amount of space on the circuit board contained within the radiotelephone. Since the keypads interact directly with the circuit board, back-lighting each individual key is necessary because there is no use of reflective plates or light pipes, this increases the power consumption and makes the keypad more cumbersome.
In the second type of design, the keypad is separated from the circuit board internal to the radiotelephone except for the electrical coupling of the keypad circuitry to the circuit board. A keypad containing 24 keys requires approximately 13 wires plus the wiring for the back-lighting circuitry. This second type of keypad supports back-lighting with reflector plates and light pipes which reduces the number of LEDs necessary to backlight a keypad. This second type of keypad also supports the use of a new technology, the new technology of conductive/resistive ink, which allows circuits to be inked onto materials other than circuit boards themselves such as thermal plastics.
As the market for radiotelephones continues to become smaller and the use of portables and personal telephones increase, the shielding of the radio transmitter from radio frequency interference (RFI) and electromagnetic interference (EMI) becomes a more important issue. There are two sources of EMI and RFI noise caused by the preceding types of keypads. The first source is the polling by the keypad circuitry which enables the radiotelephone to determine if a key has been activated. The second source of noise comes from the holes in the keypad which reduce the shielding on the radio transmitter contained within the radiotelephone. In the first keypad design the holes are for the keys themselves and in the second design there is a hole to run the wires which couple the keypad to the circuit board internal to the radiotelephone.
Although the two preceding designs have served the industry well to this point, the need exists for a keypad which is more efficient in the use of the circuit board internal to the radiotelephone, reduces the amount of RFI and EMI introduced by a radiotelephone and reduces the number of wires coupled between the keypad circuit and the circuit board internal to the radiotelephone.